Graphite material

ABSTRACT

A graphite material includes a plurality of graphite particles and a plurality of pores. The plurality of graphite particles and the plurality of pores form a microstructure. A ratio of an elastic modulus to a compression strength of the graphite material ranges from 109 to 138. Preferably, a ratio of a total area of the pores to a whole area of the graphite material in a cross-section of the graphite material ranges from 17.94% to 19.97%.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of the U.S. patentapplication Ser. No. 12/132,057 filed on Jun. 3, 2008, which claimspriority under 35 U.S.C. §119 to Japanese Patent Application No.2007-151661, filed on Jun. 7, 2007, and priority from Japanese PatentApplication No. 2008-092704, filed on Mar. 31, 2008, the contents ofwhich are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a graphite material.

2. Discussion of the Background

Since graphite materials are excellent in chemical stability, thermalresistance, processing characteristic, and the like, the materials havebeen used in many fields including an electrode for electricdischarging, a jig for glass-sealing and brazing of electronic parts andan elastic body. Recently, with miniaturization of home electricappliances and automobile parts, precise processing to form thin ribsand grooves, thin pins, fine holes, and the like is performed to moldsfor use in die-casting and plastic molding. For the purpose of preparingsuch precise molds, an electrode for electric discharging comprising agraphite material which can be precisely processed has been needed.

For obtaining a precise form such as a thin rib by electric dischargingusing a graphite material as an electrode without breaking theelectrode, it is necessary for the graphite material to have some degreeof strength. Moreover, in order to enhance dimensional accuracy of amold to be processed, it is important for the graphite material not tobe deformed by heat and external force during electric discharging.

As a high-strength and high-density graphite material suitable for suchan application, JP-A-1-97523 describes to use mesocarbon microbeads as araw material. As another means for producing a high-density andhigh-strength graphite material, JP-A-4-240022 describes to moldmesocarbon microbeads having specific β resin content, ash content,water content, volatile content, fixed carbon, and average particlediameter as a raw material under cold press, and burning and graphitizeit at a predetermined temperature. Since the graphite materials obtainedby production processes described in JP-A-1-97523 and JP-A-4-240022 havehigh strength and high density, it is advantageous that the materialsare difficult to break even when they are processed into a precise formsuch as a thin rib. The contents of JP-A-1-97523 and JP-A-4-240022 areincorporated herein by reference in their entirety.

Meanwhile, JP-A-6-144811 describes a carbonaceous coil spring in orderto remedy a disadvantage of conventional springs such as a metal springand a ceramic spring. That is, a metal spring has a large temperaturedependency in the spring constant and thus is generally used at 200° C.or lower, and its thermal resistance is also limited to 600° C. and thestrength rapidly decreases above the temperature. Moreover, the metalspring is poor in corrosion resistance against rust and chemicals. Thethermal resistance of a ceramic spring is also limited to 1000° C. andthe thermal shock resistance of the ceramic spring is poor. Since bothof metal and ceramic have high specific gravity, it is disadvantageousthat a device having the metal or ceramic spring incorporated thereinhas a large weight.

The method for obtaining the carbonaceous coil spring described inJP-A-6-144811 includes: forming an organic material capable ofcarbonization or an organic string body, which contains carbon fibers,graphite whiskers, graphite powders, amorphous carbon powders, or thelike homogeneously dispersed therein and is highly reinforced, into acoil shape; subjecting it to a carbon precursor treatment as needed;carbonizing it through a heating treatment in an inert atmosphere; andcovering the whole surface of the carbonized spring with a metalcorresponding to a desired function. The carbonaceous coil spring hasexcellent thermal resistance and corrosion resistance even at a hightemperature in the presence of oxygen and is expected to have highstrength and reliability.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a graphite materialincludes a plurality of graphite particles and a plurality of pores. Theplurality of graphite particles and the plurality of pores form amicrostructure. A ratio of an elastic modulus to a compression strengthof the graphite material ranges from 109 to 138.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A shows a graph of particle size distribution of a secondary rawmaterial powder used in Example 1;

FIG. 1B shows values of particle size distribution of the secondary rawmaterial powder used in Example 1;

FIG. 2A shows a graph of particle size distribution of a secondary rawmaterial powder used in Example 2;

FIG. 2B shows values of particle size distribution of the secondary rawmaterial powder used in Example 2;

FIG. 3A shows a graph of particle size distribution of a secondary rawmaterial powder used in Comparative Example 1;

FIG. 3B shows values of particle size distribution of the secondary rawmaterial powder used in Comparative Example 1;

FIG. 4A shows a graph of particle size distribution of a secondary rawmaterial powder used in Comparative Example 2;

FIG. 4B shows values of particle size distribution of the secondary rawmaterial powder used in Comparative Example 2;

FIG. 5A shows a cross-sectional SEM photograph of the graphite materialprepared in Example 1;

FIG. 5B shows a binarized image obtained by image-processing of thecross-sectional SEM photograph of the graphite material prepared inExample 1;

FIG. 5C shows an elliptic fitting drawing of a binarized image obtainedby image-processing of the cross-sectional SEM photograph of thegraphite material prepared in Example 1;

FIG. 6A shows a cross-sectional SEM photograph of the graphite materialprepared in Example 2;

FIG. 6B shows a binarized image obtained by image-processing of thecross-sectional SEM photograph of the graphite material prepared inExample 2;

FIG. 6C shows an elliptic fitting drawing of a binarized image obtainedby image-processing of the cross-sectional SEM photograph of thegraphite material prepared in Example 2;

FIG. 7A shows a cross-sectional SEM photograph of the graphite materialprepared in Comparative Example 1;

FIG. 7B shows a binarized image obtained by image-processing of thecross-sectional SEM photograph of the graphite material prepared inComparative Example 1;

FIG. 7C shows an elliptic fitting drawing of a binarized image obtainedby image-processing of the cross-sectional SEM photograph of thegraphite material prepared in Comparative Example 1;

FIG. 8A shows a cross-sectional SEM photograph of the graphite materialprepared in Comparative Example 2;

FIG. 8B shows a binarized image obtained by image-processing of thecross-sectional SEM photograph of the graphite material prepared inComparative Example 2;

FIG. 8C shows an elliptic fitting drawing of a binarized image obtainedby image-processing of the cross-sectional SEM photograph of thegraphite material prepared in Comparative Example 2;

FIG. 9 shows a perspective view of an elastic body made of a graphitematerial;

FIG. 10 shows an example of a lathe used in producing the elastic bodymade of a graphite material; and

FIGS. 11A to 11E show diagrams of a process for producing the elasticbody made of a graphite material.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

First Embodiment

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings. The following willdescribe embodiments of the graphite material according to the presentinvention in detail.

Since conventional graphite materials have high strength and highdensity, cutting resistance with a cutting tool during processing islarge and chipping often occurs. Moreover, since the cutting resistancewith a cutting tool is high, in the case of processing a thin rib and afine pin, the graphite material is deformed by a reaction force, andtherefore, accuracy in thickness decreases. Furthermore, also in thecase where inner faces or bottom faces of small frames having a smallcorner R, thin grooves, deep fine holes, and the like are processedusing an end mill or a drill, the end mill or drill deforms, andtherefore, not only processing of high accuracy cannot be achieved butalso breakage of the cutting tool is caused.

The prevention of these problems is principally possible by decreasingan amount of one cutting by a cutting tool. However, for doing this, itis necessary to take a countermeasure, that is, decrease an advancingrate of the cutting tool or increase rotation number of the cuttingtool. In such a method, it is necessary to use high-performanceprocessing machine and cutting tool having a high rigidity which doesnot generate sway of the center even at high-speed rotation and also theprocessing takes more time.

Furthermore, in the case where a conventional graphite material is usedas an electrode for electric discharging for finish processing, agraphite material generally has a relation that consume of the electrodedecreases as shore hardness thereof increases. Therefore, a graphitematerial obtained at a low graphitization temperature and having a highshore hardness is advantageous. However, a graphite material having ahigh shore hardness also has high cutting resistance and rapidlyconsumes the cutting tool.

Meanwhile, in the case of the conventional carbonaceous coil spring, itis difficult to form a spring with a high accuracy since the process ofcarbonization of the organic string body involves a size contraction.Moreover, since the carbon material formed by such a method is a glassycarbon having a high hardness, it is difficult to set its shape bypost-processing. It is noted that it is conceivable to process widelyused isotropic graphite material into a predetermined shape such as acoil shape to make a spring. However, since pores in the isotropicgraphite material is generally flat and large, a crack may easilydevelop from an edge part of the flat pores to lead to breakage of thespring, so that the widely used isotropic graphite material is notsuitable as material for a spring.

According to an embodiment of the present invention, a graphite materialand a method for producing such a graphite material may be provided,which has a high strength and a high density and also excellent inprocessing characteristic.

As a result of extensive studies in consideration of the above andaccording to the embodiment of the present invention, a graphitematerial having a specific structure enables processing with goodaccuracy without breaking cutting tools in the precise processing ofthin ribs, thin pins, narrow grooves, fine holes, and the like.

The graphite material according to the above has a fine structurewherein the number of the pores appearing on a cross-section is 250 ormore per 6000 μm², average area of the pores appearing on thecross-section is 5 μm² or less, and average aspect ratio of the poresappearing on the cross-section is 0.55 or less as well as fine graphiteparticles and the pores are homogeneously distributed. Therefore, thematerial has a high strength and a high elastic modulus as well asexcellent in processing characteristic. Therefore, at the time whenprecise processing for a thin rib and the like is performed using thegraphite material according to the above as an electrode for electricdischarging, processing can be performed with good accuracy withoutbreaking the graphite material and cutting tools. Moreover, since thegraphite material according to the above enables fine processing andalso attrition at the electric discharging is small, a mold having afine pattern can be easily produced and thus the material isadvantageous as an electrode for electric discharging in finishprocessing.

According to an embodiment of the present invention, there is provided agraphite material including a plurality of graphite particles and aplurality of pores which form a microstructure. When a cross-section ofthe microstructure is observed with a scanning electron microscope, thenumber of the pores appearing on the cross-section is 250 or more per6000 μm², and an average area of the pores appearing on thecross-section is 5 μm² or less. According to the above, the poresdistributed in the graphite material are sufficiently small and thenumber of the pores present per unit volume of the graphite material issufficiently large. Therefore, chipping in a large particle unit doesnot occur and a smooth processed surface can be obtained. Moreover,since the pores are very small as compared with the usual processed formapplied to the graphite material, occurrence of fracture caused by thechipping of particles in the thin pin processing and crack and boring inthe thin rib cutting can be reduced.

Additionally, when the cross-section of the microstructure is observedwith the scanning electron microscope, an average aspect ratio of thepores appearing on the cross-section is 0.55 or less. According to this,the elastic modulus of the graphite material increases relative tocompression strength by a cutting tool at processing. Therefore, thesize of cutting chips generated at the processing can be reduced.Namely, cutting resistance of the cutting tool is small and thusprocessing becomes easy.

The above relation between the form of pores of the graphite materialand the processing characteristics thereof is surmised to be due to thefollowing mechanism.

At the cutting of a graphite material, compression force acts thereon inan advancing direction of a cutting tool. On this occasion, when strainenergy accumulated with the advance of the cutting tool exceeds energynecessary for breakage, the material is cut. In order to obtain a smoothprocessed surface, it is necessary to process the material withdischarging fine cutting powder and the occurrence of breakage isimportant prior to the accumulation of large strain energy.

In order not to accumulate the large strain energy, a small compressionstrength and a large elastic modulus are necessary. It is pointed outthat the diameter of the particles to be cut has a positive correlationwith the value of (compression strength)/(elastic modulus). From theabove, in order to obtain a processed surface having a small (fine)particle diameter of particles to be cut, it is realized that a graphitematerial having a larger elastic modulus is advantageous.

The following will describe the relation between the elastic modulus ofthe graphite material and the shape of the pores. In general, theelastic modulus of a graphite material is represented by the followingKnudsen's empirical equation:

E(P)=E(0)exp(−bP)

where E(P): elastic modulus, P: porosity, and b: empirical constant.

The empirical constant b highly depends on the shape of the pores and itis known that the value is small in the case where the shape of thepores is spherical and the value rapidly increases as the shape changesthrough a flattened spheroid into a cracked pore shape (“Shin-TansoZairyo Nyumon” (Guide to Carbon Material), edited by The Carbon Societyof Japan). Therefore, in order to increase the elastic modulus, agraphite material having a round shape (small aspect ratio) isadvantageous.

From the above, it is considered that the relation between the shape ofthe pores of the graphite material and its processing characteristic isintroduced. Namely, since the elastic modulus of the graphite materialcan be increased by making the shape of the pores round, i.e., theaverage aspect ratio of the pores appearing on the observedcross-section is made 0.55 or less, a fine-grained processed surface canbe obtained, and therefore, a graphite material excellent in processingcharacteristic can be obtained.

Then, with regard to the compression strength, even when the pores areflattened spheroids or cracked pores, applied compression load acts soas to crash the pores, so that the shape of the pores does not affectthe compression strength. The porosity affects the compression strengthmore.

When the porosity is small, the compression strength becomes high, andtherefore, it becomes hard to cut the material and unevenness on theprocessed surface increases. When the porosity is large, the compressionstrength can be lowered but the resulting graphite material becomes softso that it is easily broken or cracked even when fine processing isapplied. Moreover, it is easily attrited in the electric discharging.

The porosity of the graphite material is highly correlated with its bulkdensity. In the case where the same raw material is used and it issubjected to the same graphitization treatment, the bulk density isabout the same when the porosity is the same.

In the embodiment of the present invention, since pitch is mainly usedas a starting material and the starting material and graphitizationtemperature fall within limited ranges although component(s) transformedthrough pitch cokes and component(s) directly carbonized and graphitizedexist, the bulk density of the graphite material becomes from 1.78 to1.86 g/cm³, preferably 1.82 to 1.85 g/cm³. In this connection, the bulkdensity according to the embodiment of present invention is obtained bymeasuring the volume and weight of the material.

In the embodiment of the present invention, the number, average area,and average aspect ratio of the pores appearing on the cross-section canbe calculated by observing the graphite material with an electronmicroscope or the like. Specifically, the cross-section of the graphitematerial is processed by a cross-section polisher (CP) method. After theprepared cross-section is subjected to a flat milling treatment (45°, 3minutes), the number, average area, and average aspect ratio of thepores are obtained by observing the cross-section with an FE-SEM.

At analysis of the obtained images, after binarization using an imageanalyzing software (IMAGE J 1.37), the area of each void (pore appearingon the cross-section) is calculated. The average area is obtained bydividing the total area of the pores by the number of pores. And,elliptic fitting is performed on each void and an aspect ratio iscalculated based on the values of the major axis and the minor axisthereof.

In this connection, the aspect ratio means a value of(major-axis-minor-axis)/(major-axis) of the ellipse fitted void (poreappearing on the cross-section).

At the measurement of the number, average area, and average aspect ratioof the pores appearing on the cross-section, SEM is used as mentionedabove. This is because a sufficient resolution for determining the shapeof pores in a micron order can be obtained and also the pores and theparticles can be clearly discriminated. The particle part is displayedas gray having a single density and the pore part is displayed as blackin the case of deep pores and as white in the case of shallow pores,depending on the depth of the pores.

At the measurement of the number, average area, and average aspect ratioof the pores appearing on the cross-section, it may be advantageous touse a graphite material which is not filled with a resin. This isbecause, when the graphite material is filled with a resin, the openpores present inside the graphite material are sealed with the resin andthus correct number and shape of the pores cannot be determined.

The maximum pore diameter (major-axis or maximum size of the pores) ispreferably 20 μm or less. When the maximum pore diameter is more than 20μm, crack is developed along the pores at cutting, so that a thin pin isbroken and a thin rib is cracked at cutting processing, which causeshole formation.

The maximum pore size (diameter) can be also measured from thecross-section observed with SEM in the same manner as described above.In this connection, the diameter of the pore obtained from the SEMobservation of the cross-section is different from the diameter of thepore and the graphite particle obtained by means of a mercuryporosimeter or the like. The former measures an actual size but thelatter measures a diameter of the entrance part of a continuous pore.

The Shore hardness of the graphite material according to the embodimentof the present invention is preferably from 55 to 80. When the Shorehardness is less than 55, the chipping of the particles increases duringelectric discharging and attrition of the electrode becomes large, sothat the resulting material is not suitable as an electrode for electricdischarge. When the Shore hardness exceeds 80, the cutting resistancewith a cutting tool increases at the time of cutting processing of anelectrode, so that attrition of the cutting tool occurs rapidly and alsothe material might be easily broken or chipped.

The Shore hardness can be measured in accordance with Japan IndustryStandards (JIS) Z2246. The contents of Japan Industry Standards (JIS)Z2246 are incorporated herein by reference in their entirety.

The specific resistance of the graphite material according to theembodiment of the present invention is preferably from 1000 to 2300μΩcm, more preferably, 1000 to 2000 μΩcm. The specific resistancecorrelates with the Shore hardness of the graphite material and when thespecific resistance is lowered, the graphite material is softened. Inthe case where the specific resistance is less than 1000 μΩcm, the Shorehardness becomes less than 55 and the attrition of the electrode becomeslarge. In this case, even when the material is processed in a finepattern and used as an electrode, the processed accuracy cannot betransferred to a mold owing to severe attrition of the electrode. In thecase where the specific resistance is more than 2300 μΩcm, when thematerial is used as an electrode for electric discharge, abnormalelectric discharge may occur and unevenness tends to be generated on theprocessed surface of an article to be processed.

The specific resistance can be measured in accordance with JIS R7222, afall-of-potential method. The contents of JIS R7222 are incorporatedherein by reference in their entirety.

The graphite material according to the embodiment of the presentinvention can be suitably used especially as an electrode for electricdischarging for finish processing. In rough processing, a mold isroughly processed and particularly fine processing is not provided. Thegraphite material according to the embodiment of the present inventioncan be processed in a fine and highly accurate pattern necessary infinal finish processing.

The following will describe the process for producing the graphitematerial according to the embodiment of the present invention. Theprocess for producing the graphite material according to the aspect,including adding a carbonaceous fine powder to pitch, kneading (mixing)them, performing thermal treatment (heating) at 400 to 500° C. while avolatile content is controlled to obtain a secondary raw material.Further, the process comprises pulverizing the obtained secondary rawmaterial while controlling particle size so as not to over-pulverize itin a pulverizing machine having a function of removing a fine powderhaving smaller particle diameter, thereby a secondary raw materialpowder (particles) is obtained, and thereafter, molding the secondaryraw material powder into a cuboid by cold isostatic press molding (CIPmolding), burning at about 1000° C. in a burning furnace, andgraphitizing at about 2500° C. in a graphitization furnace to obtain agraphite material according to the embodiment of the present invention.

The pitch for use in the embodiment of the present invention means acarboniferous or petroleum pitch and may be a mixture thereof. Of theseraw materials, it may be advantageous to use a carboniferous pitch. Inthe case of the carboniferous pitch, optical anisotropy is difficult todevelop (crystals are difficult to develop into a needle shape) and ahigh-strength and high-elastic modulus graphite material can beobtained.

The softening point of the pitch for use in the embodiment of thepresent invention may be preferably 50° C. or lower. When the softeningpoint is higher than 50° C., the viscosity at the kneading increases andthe production becomes difficult.

The carbonaceous fine powder for use in the embodiment of the presentinvention becomes a nucleus at development of a meso-phase and there canbe used carbonaceous one such as carbon black, a graphite fine powder, araw pitch coke fine powder, or a calcined pitch coke fine powder. Thesize of the fine powder may be preferably 5 μm or less. When a finepowder is more than 5 μm, it becomes difficult to control particle sizedistribution at pulverization of the secondary raw material obtained bykneading and a coarse side of the particle size distribution increases.The amount to be added to the pitch may be preferably from 3 to 10% byweight. When the fine powder is added in an amount more than 10% byweight, the viscosity of the pitch increases and the production becomesdifficult. When the amount is less than 3% by weight, a mosaic structureof cokes cannot be sufficiently developed.

In the thermal treatment of the raw material described above,temperature and time are controlled so that the volatile contentmeasured by JIS 8812 becomes from 6 to 12%, more preferably from 8 to11%, thereby the secondary raw material is obtained. When the volatilecontent is less than 6%, since adhesion between the particles cannot besufficiently obtained, only a graphite material having a low density canbe obtained. When the volatile content is more than 12%, the amount ofhydrocarbon gas generated from the inside during burning is large, sothat the resulting material tends to be broken and also the accumulatedgas forms large pores. The contents of JIS 8812 are incorporated hereinby reference in their entirety.

The secondary raw material obtained by thermal treatment of the rawmaterial described above is pulverized while controlling the particlesize and fine powder has been removed from the resulting secondary rawmaterial powder. The method for pulverization includes a method of usinga pulverizing machine including classifying machine therein, a method ofusing a pulverizing plant including a pulverizing machine and a preciseairflow classifying machine, a method of separately controlling particlesize of a raw material, which has been pulverized in a pulverizingmachine, in a precise airflow classifying machine, and the like.

In a graphite material using a secondary raw material powder containinga fine powder, gas generated during burning is difficult to release andthe material tends to be broken. Furthermore, gas is accumulated in thematerial to form large pores.

With regard to the secondary raw material powder, median size (DP-50: adiameter at integral number of 50%) measured by means of a laserdiffraction-type particle size meter is preferably from 5 to 10 μm, morepreferably from 6 to 9 μm. Usually, the pores present between particlesare frequently pores having a sharp edge and a large aspect ratio. Inthe case where the size of the particles is large, the size and shape ofthe pores show a synergistic effect and causes large decrease in elasticmodulus. When the median size is more than 10 μm, the elastic modulusdecreases and the graphite material according to the embodiment of thepresent invention cannot be obtained. Moreover, when the median size isless than 5 μm, the volatile content generated from a molded article ofthe secondary raw material powder during burning cannot be rapidlydischarged to the outside of the material and thus the material tends tobe broken. Furthermore, gas is accumulated in the material to form largepores.

Moreover, with regard to the secondary raw material powder, the range ofthe particle size distribution measured by means of a laserdiffraction-type particle size meter is preferably from 1 μm to 80 μm.When the raw material of less than 1 μm is contained, the volatilecontent generated from a molded article of the secondary raw materialpowder during burning cannot be rapidly discharged to the outside of thematerial and thus the material tends to be broken. Furthermore, gas isaccumulated in the material to form large pores. When particles of 80 μmor more are contained, flattened pores tends to be formed at an outerperipheral part of large particles and in the vicinity of interface oflarge particles themselves as well as the number of the pores decreasesand the average cross-sectional area also decreases.

As the laser diffraction-type particle size meter, for example, LA-750manufactured by HORIBA LTD. can be employed. In the measurement, thesecondary raw material is dispersed by surface active agents such astween 20.

The following will describe aspects of the present invention further indetail with reference to Examples but the present invention is notlimited to the following Examples.

EXAMPLES 1. Production of Graphite Material Examples 1 and 2

To 95 parts by weight of a carboniferous pitch having a softening pointof 40° C. was added 5 parts by weight of calcined cokes pulverized intoan average diameter of 2 μm, and the whole was kneaded. Thereafter, itwas subjected to thermal treatment and the volatile content was adjustedunder thermal treatment at 415° C. to obtain a secondary raw material.Then, the secondary raw material was pulverized by means of apulverizing machine having an internal classifying machine so as not toreach over-pulverization, thereby a secondary raw material powder wasobtained. Subsequently, after pressurization was performed at a pressureof 100 MPa by means of an isostatic press, the powder was burned to1000° C. at a temperature-increasing rate of about 5° C./hour andgraphitization was carried out at 2500° C.

In this connection, the secondary raw material powder obtained in theprogress of the production did not contain powders having a diameter of1 μm or less and powders having a diameter of 80 μm or more in aparticle size distribution measured on a laser diffraction-type particlesize distribution meter.

Table 1 shows characteristic values of the raw materials used and Tables2 and 3 show characteristic values of the graphite materials obtained.

Comparative Example 1

A graphite material was produced in the same manner as in Examples 1 and2 except that the pulverization was carried out by means of apulverizing machine having no internal classifying machine. In thisconnection, the secondary raw material powder obtained in the progressof the production was not subjected to an operation of precise airflowclassification or the like and did not contain powders having a diameterof 80 μm or more but contained powders having a diameter of 1 μm or lessin an amount of 9.3% in a particle size distribution measured on a laserdiffraction-type particle size distribution meter.

Table 1 shows characteristic values of the raw materials used and Tables2 and 3 show characteristic values of the graphite materials obtained.

Comparative Example 2

To 35 parts by weight of a carboniferous pitch having a softening pointof 80° C. was added 65 parts by weight of calcined cokes pulverized intoan average diameter of 14 μm, and the whole was kneaded. Thereafter, itwas subjected to thermal treatment and the volatile content was adjustedunder thermal treatment at 250° C. to obtain a secondary raw material.Then, it was pulverized by means of a pulverizing plant fitted with apulverizing machine and a precise airflow classification machine so asnot to reach over-pulverization, thereby a secondary raw material powderwas obtained. Subsequently, after pressurization was performed at apressure of 100 MPa by means of an isostatic press, the powder wasburned to 1000° C. at a temperature-increasing rate of about 5° C./hourand graphitization was carried out at 2500° C.

In this connection, the secondary raw material powder obtained in theprogress of the production did not contain powders having a diameter of1 μm or less but contained powders having a diameter of 80 μm or more inan amount of about 3% in a particle size distribution measured on alaser diffraction-type particle size distribution meter.

Table 1 shows characteristic values of the raw materials used and Tables2 and 3 show characteristic values of the graphite materials obtained.

2. Characterization

The following items were measured to characterize the graphite materialsobtained in the above.

(Bulk Density, Shore Hardness, Specific Resistance)

Test pieces having a size of φ8×60 mm were cut out of the graphitematerials prepared in the above and the bulk density, Shore hardness,and specific resistance were measured and/or calculated according to theabove methods.

(Number, Average Area, Average Aspect Ratio of Pores Appearing onCross-Section)

The number, average area, and average aspect ratio of the poresappearing a cross-section were calculated by the following procedures.

(a) Rough Grinding of Sample

The test piece prepared in the above was cut into a column having athickness of about 5 mm and both surfaces of the sample weresurface-fixed using a jig MODEL 623 manufactured by GATAN, INC. and aSiC water-proof abrasive paper #2400. Then, the sample was fixed on abrass sample table.

(b) CP Processing

CP processing was performed at an accelerating voltage of 6 kV usingSM09010 manufactured by JEOL LTD.

(c) Milling

Ar milling treatment was performed at an accelerating voltage of 5 kV,0.5 mA, a sample tilt angle of 45°, and a milling time of 3 minutesusing a flat milling apparatus E-3200 manufactured by HITACHIHIGH-TECHNOLOGIES CORPORATION.

(d) FE-SEM Observation

The sample prepared as above was observed at an accelerating voltage of2 kV using an ultra-high resolution field emission-type scanningelectron microscopy S-4800 manufactured by Hitachi High-TechnologiesCorporation.

(e) Image Analysis

The SEM image obtained in the above was analyzed using an analyzingsoftware Image J 1.37 manufactured by National Institutes of Health. Theobserving magnification on this occasion was 2000-fold and, afternoise-reduction treatment, binarization into the planer parts/void(pore) parts was performed. In this connection, the voids (pores) to betargeted for the analysis were those having a size exceeding 0.2 μm onwhich determination whether they were voids (pores) or not is possible.

On the void (pore) parts obtained by binarization using the imageanalyzing software (Image J), area measurement and optimum ellipsefitting were carried out and also the number was counted. Then, thenumber, average area, and average aspect ratio of the pores appearing onthe cross-section were calculated from the values obtained by the abovetreatment.

(Compression Strength)

Measurement was performed in accordance with JIS R7222. The contents ofJIS R7222 are incorporated herein by reference in their entirety.

(Elastic Modulus)

Measurement was performed in accordance with JIS R7222.

3. Performance Evaluation Test

The graphite material obtained in each of Examples and ComparativeExamples was processed into a rod having a size of about φ70×100 mm. Theprocessing was performed on a lathe at a cutting depth of 1 mm and anadvancing rate of 1 mm/rotation. The number of rotations of the lathewas set at 120 rpm. As a cutting tool, TNGG160408R-A3 manufactured byKYOCERA Corporation was used.

Thus obtained cutting chips were collected and applied to a multistagevibrating sieve and median size (DP-50: a diameter at integral number of50%) was measured. In this connection, it is difficult to obtain anaccurate value of median size by means of the multistage vibrating sievesince the number of sieves usable is limited but a value of median sizewas obtained by interpolation from the passed amount toward the mesh ofthe lowest sieve through which 50% by weight of the chips were passedand the passed amount toward the mesh of the highest sieve through which50% by weight of the chips could not be passed. The processingcharacteristic of the graphite material was evaluated based on theobtained DP-50 values. It is determined that one having a smaller valuethereof is excellent in processing characteristic and exhibits lesscracking and chipping. Table 3 shows evaluation results of processingcharacteristic on the samples of Examples and Comparative Examples.

TABLE 1 Softening Median size of % by % of volatile Median size ofMinimum particle Maximum particle point of Carbonaceous carbonaceousweight of content of second secondary raw size of secondary size ofsecondary pitch fine powder fine powder pitch raw material powder rawpowder raw powder Example 1 40° C. Pitch cokes 2 μm 95% 10.3 7.3 μm 1.5μm  30 μm Example 2 40° C. Pitch cokes 2 μm 95% 9.8 7.7 μm 1.5 μm  34 μμComparative 40° C. Pitch cokes 2 μm 95% 10.3 4.0 μm 0.1 μm  26 μmExample 1 Comparative 80° C. Pitch cokes 14 μm  35% 12.0  22 μm 1.5 μm200 μm Example 2

TABLE 2 Number Ratio of total Average Average aspect of pores/ area ofpores area of ratio of 6000 μm² to whole (%) pores μm² fitted ellipseExample 1 364 17.94 2.97 0.50 Example 2 336 19.97 3.58 0.50 Comparative228 24.35 6.57 0.41 Example 1 Comparative 44 8.70 11.95 0.64 Example 2

TABLE 3 Bulk Elastic Specific Processing density Compression modulusShore resistance characteristic g/cm³ strength MPa GPa hardness μΩcm μmExample 1 1.85 92 12.7 67 1850 820 Example 2 1.82 105 11.4 63 1790 890Comparative 1.79 128 11.2 68 1520 1010 Example 1 Comparative 1.76 83 8.257 1630 1310 Example 2

As shown in Table 3, since the graphite materials of Examples 1 and 2belonging to the range of the embodiment of the present invention resultin small cutting chips as compared with Comparative Examples 1 and 2, itis realized that more precise processing is possible and thus they areexcellent in processing characteristic.

Moreover, from the cross-sectional photographs shown in FIGS. 5A to 5Cand FIGS. 6A to 6C, it is realized that a large number of round poresrelatively small in size are homogeneously distributed in the graphitematerials according to the embodiment of the present invention.Contrarily, it is realized that the number of round pores are small anda large number of relatively large pores are present in the graphitematerials of Comparative Examples shown in FIGS. 7A to 7C and FIGS. 8Ato 8C.

The graphite materials according to the embodiment of the presentinvention hardly generate cracking, chipping, and the like even whenfine processing is applied. Thus, the graphite material can be utilizedas electrodes for electric discharging having fine patterns, fine holes,pins, ribs, or the like, jigs for electronic parts, elastic bodies, andthe like.

Second Embodiment

The following will describe an elastic body which is an exemplaryapplication of the graphite material according to one aspect of thepresent invention. The elastic body made of the graphite material issuitable for use in various devices for chemical synthesis, aerospaceenvironment utilizing devices, nuclear reactors, nuclear fusionreactors, high-temperature furnaces for thermal treatment, sensors,differential thermal balances, chemical pumps, parts for engines.Particularly, in the case where the elastic body made of the graphitematerial according to one aspect of the present invention has aplate-shape, the elastic body made of the graphite material may beapplied with a load in a thickness direction thereof and may be used as,for example, a diaphragm, a leaf spring, a conical spring, and the likein a pressure sensor, a load cell, and the like. In the case where anelastic body made of the graphite material has a string-shape, theelastic body made of the graphite material may be applied with a load ina thickness direction thereof or in a twisting direction thereof, mayhave not only a linear-shape but also a spiral-shape, and may be used asa coil spring, a flat-coiled spring, and the like.

FIG. 9 shows a perspective view of the elastic body made of a graphitematerial. Hereinafter, a coil spring 11 will be described as an exampleof the elastic body made of the graphite material according to oneaspect of the present invention. The coil spring 11 is obtained bycutting (carving) an outer periphery 13 a of a cylindrical spring basematerial 13 made of a graphite material with a spiral cutting groove 15to form a coil spring shape as an axis line L being centered. Namely,the coil spring 11 is formed into a coil spring shape in which a rodhaving a square cross-section is spirally wound. In the usual coilspring formed by winding a rod, an edge part (seat) 13 b should beprocessed into flat. However, in the case of the coil spring 11, sincethe flat cylinder edge part 13 b of the cylindrical spring base material13 can be utilized as it is, the flattening process can be easilyperformed. In this connection, if the cylindrical spring base material13 is formed in a cone shape, a conical coil spring can be obtained by asimilar manner.

The following will describe the process for producing the coil spring11. FIG. 10 shows an example of a lathe used in producing the elasticbody made of a graphite material. FIGS. 11A to 11E show diagrams of aprocess for producing the elastic body made of a graphite material. Theprocess for producing the elastic body made of the graphite materialinvolves the production of the cylindrical spring base material 13 whichis made of the graphite material as shown in FIG. 11A. It is noted thatthe graphite material itself is produced as explained in the firstembodiment.

As shown in FIG. 11B, a columnar body 17 is fixed to an inner peripheryof the cylindrical spring base material 13 with an adhesive to obtain aworkpiece W1. The columnar body 17 may be made of a graphite material.Any adhesive may be used which is thermally decomposable andvaporizable. For example, α-cyanoacrylate adhesive (instant adhesive) ispreferably used. The α-cyanoacrylate adhesive is depolymerized intomonomers by heating to the range from 200 to 300° C. Therefore, theadhesive can be thermally decomposed without oxidizing the graphitematerial since an oxidization onset temperature of the graphite materialis around 400° C.

Then, using a lathe 19 shown in FIG. 10, while rotating the workpiece W1about the axis line L, a cutting tool (turning tool) 21 is relativelydisplaced in parallel to the axis line L to cut the cylindrical springbase material 13 with a spiral groove 23 which reaches the columnar body17, as shown in FIG. 11C. Specifically, as if the screw-thread cuttingwould be performed on the workpiece W1, the workpiece W1 is rotated witha main axis 25 as a rotation center. The cutting tool 21 is displacedfrom a cutting tool holder 27 along a guide axis 31 parallel to the mainaxis 25 with synchronizing the rotation of the workpiece W1 while thecutting tool 21 is brought into contact with a peripheral of theworkpiece W1. Accordingly, the spiral groove 23 is formed. In thisconnection, the columnar body 17 serves as a reinforcing member of thecylindrical spring base material 13 and strength against crushing towardthe inside of the cylindrical spring base material 13 in a radialdirection is enhanced, so that it becomes possible to perform a spiralgroove cutting processing to the outer periphery 13 a of the cylindricalspring base material 13.

After a workpiece W2 having the spiral groove 23 formed as shown in FIG.11D is obtained, the workpiece W2 with the spiral groove 23 is thensubjected to a thermal treatment at a temperature ranging from thedecomposition temperature of the adhesive or higher to the oxidationtemperature of the graphite material or lower. Then, the columnar body17 is removed. Thus, the coil spring 11 shown in FIG. 11E is produced.

Consequently, according to the coil spring 11, it is formed using agraphite material comprising: a plurality of graphite particles; and aplurality of pores. The plurality of graphite particles and theplurality of pores form a microstructure, and a cross-section of themicrostructure has a number of the pores that is 250 or more per 6000μm², an average area of the pores on the cross-section is 5 μm² or less,and an average aspect ratio of the pores on the cross-section is 0.55 orless. Therefore, fine graphite particles and pores are homogeneouslydistributed and the elastic body has thermal resistance, corrosionresistance, and cutting ability with a high strength and a high elasticmodulus and further, dimensional accuracy can be enhanced. As a result,the coil spring 11 remedies the defect of the carbon material, does notbroken even after repeated use in various devices for chemicalsynthesis, aerospace environment-utilizing devices, nuclear reactors,nuclear fusion reactors, and the like, can be also stably utilized undera situation where a metal spring cannot be used, and have a longoperating life.

Moreover, the method for producing the coil spring 11 includes: forminga cylindrical spring base material 13 using the above graphite material;obtaining a workpiece by fixing a columnar body, to an inner peripheryof the cylindrical spring base material 13 with an adhesive; relativelydisplacing a cutting tool in parallel to a center axis of thecylindrical spring base material 13 while rotating the workpiece W1about the center axis to cut the cylindrical spring base material 13with a spiral groove 23 which reaches the columnar body 17; heating theworkpiece W2 cut with the spiral groove 23 to depolymerize the adhesive;and removing the columnar body 17 from the cylindrical spring basematerial 13. Therefore, the spiral groove cutting processing can beperformed to the outer periphery 13 a of the cylindrical spring basematerial 13 while using the columnar body 17 as a reinforcing member ofthe cylindrical spring base material 13 without crushing the cylindricalspring base material 13 inward in a radial direction to thereby obtainthe elastic body made of the graphite material having a coil shape.

The following examples provide a more detailed description of aspects ofthe present invention. The present invention is, however, not limited tothe following examples. In the second embodiment, the Examples 1 and 2,and Comparative Examples 1 and 2 described in the first embodiment areused for producing a coil spring. Therefore, the details of theseexamples of the graphite material will be omitted.

Examples 1. Production of Coil Spring

A graphite material of each of Examples and Comparative Examples isprocessed into a hollow cylindrical shape having a thickness of 2.5 mm,which is used as a cylindrical spring base material 13 (FIG. 11A). Acolumnar body 17 is adhered to an inner periphery of the cylindricalspring base material 13 with a-cyanoacrylate to form a workpiece W1wherein the cylindrical spring base material 13 and the columnar body 17are integrated (FIG. 11B). Using a lathe 19 shown in FIG. 10, a spiralgroove 23 having a width of 1 mm and a pitch of 2 mm is formed at theworkpiece W1 (FIG. 11C). The resultant workpiece W2 is subjected to athermal treatment at 330° C. and then the columnar body 17 is removed(FIG. 11D). Accordingly, a coil spring 11 is obtained (FIG. 11E).

2. Evaluation of Coil Spring

No apparent difference was visually confirmed not only on the coilsprings of Examples (coil springs using the graphite materials ofExamples 1 and 2) but also on the coil springs of Comparative Examples(coil springs using the graphite materials of Comparative Examples 1 and2). However, as also recognized from the cross-sectional photographs ofthe graphite materials shown in FIG. 5A to FIG. 5C, FIG. 6A to FIG. 6C,FIG. 7A to FIG. 7C, and FIG. 8A to FIG. 8C, a large number of relativelysmall-sized round pores are homogeneously distributed in the graphitematerials of Examples, while round pores are small in number and a largenumber of relatively large pores are present in the graphite materialsof Comparative Examples. Therefore, the coil springs made of thegraphite materials of Examples and the coil springs made of the graphitematerials of Comparative Examples are significantly different inresistance against stress. Specifically, in the case of the coil springsof Comparative Examples, chipping occurred during the compression fromthe natural-length state to the most compressed state so that thesprings were broken only by repeating expansion and compression severaltimes. Contrarily, in the case of the coil springs of Examples, chippingdid not occur even when expansion and compression between thenatural-length state and the most compressed state were repeated, andtherefore, the springs were not broken even when the expansion andcompression were repeated 1000 times.

It should be noted that the exemplary embodiments depicted and describedherein set forth the preferred embodiments of the present invention, andare not meant to limit the scope of the claims hereto in any way.Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1. A graphite material comprising: a plurality of graphite particles; and a plurality of pores, wherein the plurality of graphite particles and the plurality of pores form a microstructure, and wherein a ratio of an elastic modulus to a compression strength of the graphite material ranges from 109 to
 138. 2. The graphite material according to claim 1, wherein a ratio of a total area of the pores to a whole area of the graphite material in a cross-section of the graphite material ranges from 17.94% to 19.97%.
 3. The graphite material according to claim 1, wherein a cross-section of the microstructure has a number of pores that is 250 or more per 6000 μm².
 4. The graphite material according to claim 1, wherein an average area of the pores on the cross-section is 5 μm² or less.
 5. The graphite material according to claim 1, wherein an average aspect ratio of the pores on the cross-section is 0.55 or less. 